Transparency with psSurface in pst-solides 3D

Question

I'm trying to draw a graph with pst-solides3D with the option psSurface :

The problem is as follows :

I want to draw z = +/- (y^2-y^2), which should give a saddle-point graph with the lines x=y and x=-y that are equal to 0. Visually it's nice.

\documentclass[a4paper,10pt]{article}
\usepackage[utf8x]{inputenc}

\usepackage[english]{babel} %change language if necessary
\usepackage[english,verbose]{layout}%change language if necessary
\usepackage[T1]{fontenc}
\usepackage{lmodern}
\usepackage{xspace}
\usepackage{pstricks}
\usepackage{pst-solides3d}
\usepackage{epstopdf}

\psset{unit=0.35} 
\psset{lightsrc=30 -10 10} 
\psset{viewpoint=50 20 30 rtp2xyz,Decran=70} 
\begin{pspicture}(-11,-8)(7,8) 
%+ Saddle graph z= (y^2-x^2)
\psSurface[ngrid=.2 .2,algebraic,Zmin=-1,Zmax=1, 
linewidth=0.5\pslinewidth,spotX=r,spotY=d, 
spotZ=l, 
hue=0 1](-5,-5)(5,5){% 
(x^2-y^2)/3 } 
%- Saddle graph z = (x^2-y^2)
\psSurface[ngrid=.2 .2,algebraic,Zmin=-1,Zmax=1, 
linewidth=0.5\pslinewidth,spotX=r,spotY=d, 
spotZ=l,
hue=0 1](-5,-5)(5,5){% 
(-x^2+y^2)/3 } 
\end{pspicture} 

However as you can see this is not satisfactory. The second plot is plotted over the first one, and covers parts of the first graph it shouldn't cover. Is there any transparency command I could use, or make pstricks understand what is in front of what ?

The rendering I'm looking for would be something like this

It knows what should be in front and what should not.

Answer 1

It makes no sense to draw more than one 3d function without using hidden lines and surfaces.

\documentclass{article}
\usepackage{pst-solides3d}

\begin{document}
\psset{unit=0.35,lightsrc=30 -10 10,viewpoint=50 20 30 rtp2xyz,Decran=70,solidmemory} 
\begin{pspicture}(-11,-8)(7,8) 
\defFunction[algebraic]{F1}(x,y){}{}{ (x^2-y^2)/3 }
\psSolid[object=surface,function=F1,ngrid=.2 .2,linewidth=0.5\pslinewidth,
  hue=0 1,base=-5 5 -5 5,action=none,name=Func1]
\defFunction[algebraic]{F2}(x,y){}{}{ (-x^2+y^2)/3 } 
\psSolid[object=surface,function=F2,ngrid=.2 .2,linewidth=0.5\pslinewidth, 
  hue=0 1,base=-5 5 -5 5,action=none,name=Func2]
\psSolid[object=fusion,base=Func1 Func2,opacity=0.6]
\end{pspicture} 

\end{document}

and the same without setting opacity (real hidden surfaces)

and if you want the color depending to the z value use

\psSolid[object=surface,function=F1,ngrid=.2 .2,
         linewidth=0.5\pslinewidth,
  
         tablez=-5 0.2 5 {} for, zcolor=0.66 0,
         base=-5 5 -5 5,action=none,name=Func1]